View Compilation Error

0_0_37454397_27663.cpp:21:1: error: 'unordered_set' does not name a type
 unordered_set<int> s[N];
 ^
0_0_37454397_27663.cpp: In function 'void tarjan(int, int, int)':
0_0_37454397_27663.cpp:45:21: error: 's' was not declared in this scope
                     s[cot].insert(z);
                     ^
0_0_37454397_27663.cpp:48:17: error: 's' was not declared in this scope
                 s[cot].insert(x);
                 ^
0_0_37454397_27663.cpp: In function 'int main()':
0_0_37454397_27663.cpp:66:29: error: 's' was not declared in this scope
             dcc[i].clear(), s[i].clear();
                             ^
0_0_37454397_27663.cpp:85:29: error: 's' was not declared in this scope
                         if (s[i].find(y) != s[i].end())
                             ^